The present invention relates to brazable articles, and more particularly, to a brazable in air tool insert fabricated from a tool compact, such as a polycrystalline diamond compact or a cubic boron nitride compact, having a multilayer coating deposited thereon. The multilayer coating forms a bond in excess of 10 Kpsi to standard brazing compounds at temperatures as low as 700.degree. C. in an air environment.
Compacts, such as polycrystalline diamond and cubic boron nitride, are often produced by a high temperature, high pressure process that uses cobalt as a sintering aid. The compacts are attached to a cemented tungsten carbide support before sintering. Cobalt can be added to the sintering furnace, however, more commonly, the cobalt is wicked out of the cemented tungsten carbide support. The cemented tungsten carbide support provides toughness to the polycrystalline diamond or cubic boron nitride compact, which are both hard but not tough.
This process has several disadvantages. The tungsten carbide support is bulky. It significantly reduces the space available for sintering the compacts, so that productivity is low. Not all types of tungsten carbide supports can be sintered with the compacts in the furnace. This prior art is limited to use of only specific types of tungsten carbide supports, while tool manufacturers prefer to have flexibility in their choice of tungsten carbide supports. With the tool compact attached to the tungsten carbide support during sintering and subsequent cooling, additional stresses are also created in the tool compact because the rate of thermal expansion of tungsten carbide is different from that of the tool compact material.
Brazing of inserts to tungsten carbide supports is typically performed at temperatures between 1000.degree. and 1200.degree. C. in a controlled vacuum with materials that contain transition metals to enhance the adhesion of the braze to the tool compact.
This high temperature braze process cannot be used with polycrystalline diamond compacts because the compacts are manufactured with cobalt or other ferrous metals as sintering aids. The metals act as catalysts for the decomposition of diamond to graphite when heated above approximately 700.degree. C.
Cubic boron nitride compacts cannot be brazed at high temperatures either. At temperatures greater than 700.degree. C., the trace components added to improve performance can migrate, degrading the material.
The high temperature process has other disadvantages: it is difficult to manage in production, uses expensive and often brittle brazes, and requires expensive equipment not usually found in the manufacturing environment in which the tools are used.
There is a great need in the industry, therefore, for a tool insert that can be brazed in air with standard low temperature brazes. To enable brazing in air, a coating material is needed that will bond to the tool compact and make a good metallurgical bond to the braze in an oxygen containing environment. Such tool inserts will then be sold to tool manufacturers for brazing onto their own tungsten carbide substrates. The resulting tools could find uses in industry in such applications as tool inserts, reamers, end mills, drill bits, dressers and cutting tools. The invention is not limited only to cutting tool inserts. The same coatings and method are suitable to any application in which a diamond or boron nitride surface is to be brazed to another hard surface, particularly when air brazing is desired.
There have been numerous attempts to solve the problem of achieving an adequate bond to the tungsten carbide support at temperatures that do not result in thermal degradation of the tool compact material. Generally, they fall into one of three categories: brazes, geometric effects, and coatings or intermediate layers.
U.S. Pat. No. 4,931,363 discloses the use of brazes containing chromium for improved bonding of thermally stable polycrystalline diamond compacts to tungsten carbide tools. EP 040,267 extends the use of high temperature carbide forming brazes to diamond created by chemical vapor deposition. Each of these methods requires high temperature brazes which must be bonded in controlled vacuum environments.
Changes in geometry are the basis of the teachings in U.S. Pat. Nos. 4,649,992 and 4,661,180 and 4,767,050 and 4,784,023. These patents utilize such shapes as frusto-conical shapes, interlocking alternating ridges, and pocketed substrates. These various approaches still require the use of expensive high temperature brazes processed in vacuum.
Prior inventions also utilize coatings and interlayers. U.S. Pat. No. 4,776,862 discloses the use of materials which form transition metal carbides as coatings on powders that are then brazed. The use of materials which form transition metal carbides as coatings on tool compacts before silver brazing is disclosed in U.S. Pat. No. 5,037,704, while GB 2091763A teaches the use of these materials as additives to abrasive powders during high pressure and temperature sintering. The Japanese patent, HEI-4[1992]-201,004, discloses the use of a thick transition metal layer 1-5 mm thick between the diamond and the tool before brazing in a vacuum at typically 1100.degree. C.